The invention is related to a hydrostatic driving mechanism for the propulsion of forklifts, wheel loaders and similar vehicles having automotive type controls. More particularly, the invention relates to an improvement in automotive-type controls for hydrostatic transmissions.
With conventional automotive controls, the angle of the swashplate of the pump is usually changed depending on the engine speed applied to the pump. FIG. 1 is exemplary of this relationship in a typical pump. At about 900 rpm the pump starts delivering fluid and the maximum swashplate angle is reached at about 2000 rpm.
The potential power consumption of the pumps is often about three to four times higher than the rated power of the combustion engine. Generally, maximum load or power limiting controls have been used to limit the torque of the transmission in view of the characteristics shown in FIG. 1 and the mismatch between engine and transmission power ratings. Usually, input signals are derived from the actual speed of the engine and from the position of the throttle pedal. During the operation of the vehicle for the first time, the so-called "learned" or baseline characteristic is recorded. This gives the relation between the position of the throttle pedal and the speed of the engine at speeds ranging from low idle to high idle, as shown in FIG. 2.
Therefore, when the vehicle is traveling, it is possible to detect whether the motor is delivering or consuming power by evaluating the speed of the combustion engine and the position of the throttle pedal. The rate of deviation from the baseline or "learned" characteristic corresponds to the load. Above the baseline in FIG. 2 lies a zone where speed is dropping. Below the baseline lies a zone where the vehicle is coasting. At 10% to 15% drop of speed, the governor device attached to the throttle pedal has reached its control limit and is set to full throttle. If engine speed is decreased further, the output torque of the transmission is limited to be equivalent to the maximum torque characteristic of the engine.
A load limiting control takes over at the control limit of the governor device and increases the displacement of the pump in the hydrostatic transmission. Thus, the load of the engine is reduced and a further drop of speed, or at worst the stalling of the engine, is prevented.
To achieve automotive characteristics during braking or inching condition, conventional controls reduce the swashplate angle based on the rate of change in the position of the brake pedal (inch pedal). It is hard to find a compromise for a loaded and unloaded vehicle because the braking capacity of the engine is firmly limited to about one third of its rated torque. To protect the engine from overspeed, either the ramp for the swashplate angle is modified or the above-mentioned load sensing control is applied acting in the opposite way. An increasing number of engines are smaller and turbocharged. When such engines are used in conjunction with the displacement reducing load sensing control, less braking torque is available.
Conventional automotive controls allow the vehicle to coast by reducing the swashplate angle along a given linear ramp, thereby increasing the transmission ratio. However, it is a problem to find one ramp which is suitable to provide acceptable deceleration of the loaded and unloaded vehicle. Coasting may be comfortable and acceptable on flat ground, but may easily stall the engine in climbing conditions.
Even with power and load limiting controls, conventional drive mechanisms having hydrostatic transmissions tend to respond abruptly and aggressively to commands. Uncomfortable jerking of the vehicle may result. Most vehicle manufacturers desire drive system performance similar to a hydrodynamic fluid torque converter. Such a converter reduces transmission displacement when traveling resistance is increased. The performance of the vehicle is less aggressive, which is normally said to be more smooth and comfortable. Nearly all manufacturers feel such performance to be a must in drive systems for propelling wheel loaders.
The primary objective task of this invention is to provide a driving mechanism with a hydrostatic transmission that performs similar to a hydrodynamic fluid torque converter.
Another objective of this invention is to provide an electronic control for controlling pump displacement based on engine speed and transmission system pressure to insure that the vehicle drives smoothly.